The present invention is directed to a plug-in holder for a centering drill bit in a drilling tool. The plug-in holder has an annular groove for receiving cylindrically shaped arresting elements of a tool chuck in a drilling device. The arresting elements extend tangentially of the annular groove.
In the device disclosed in DE a 34 34 076, the tool bit is automatically centered, in a tolerance free manner, upon insertion into a conically shaped receiving aperture while in abutment with the wall of the aperture. The rotational entrainment of the tool bit occurs by means of driving or entrainment ridges in the tool bit holder which engage in grooves in the tool bit. Axial retention of the tool bit is afforded by essentially radially displaceable pins extending tangentially to the receiving aperture and shiftable into corresponding recesses, particularly into an annular groove in the tool bit.
In electrical or sanitary installations in structures, appropriate openings must be placed in the walls. Such opening can be through bores for pipes or depressions for flush mounted sockets for switches or plug-in sockets.
Such opening usually has a large diameter. To use core drill bits of the required size, larger drilling devices are needed. These drilling devices are fastened through a base plate by a vacuum or anchoring attachments to the structural component. The drilling device is secured along with the core drill bit to the base plate.
There are certain cases, however, which require a smaller diameter opening and can easily be drilled with a hand held drilling device.
The difficulty involved with a hand held drilling device is that the core drill bit runs off center when it is applied to the structural component. Accordingly, the required accuracy in the formation of the opening can not be achieved with such a core drill bit. In the state of the art as set forth in DE A 30 29 101, a hollow core drill bit is provided with a centering bit and the core drill bit has diamond grinding segments at its drilling end. The centering bit is clamped in a rear attachment of a carrier member by a locking screw.
A spirally shaped section of the centering bit extends at least for the full axial length of the core drill bit. The centering bit remains clamped in the core drill bit during the entire drilling step.
Depending on the material of the structural component being drilled, there is the disadvantage that torques of different magnitude act on the centering bit in a blow-like manner. A locking screw pressed radially against the smooth cylindrical shank of the centering bit can not assure a secure slip-free clamping of the bit in the carrier member. If the centering bit slips it can not drill into the structural component.
When installing the shank of the centering bit in the carrier member, care must be taken that a specific spacing is maintained between the drilling end face of the diamond segments and the tip of the centering bit. Continuous use of the centering bit during the drilling step requires frequent regrinding and, as a result, a continuous reduction in the length of the centering bit. Accordingly, the shank end of the centering bit can not be abutted at the base of the receiving bore in the carrier member. Therefore, there is the danger that the centering bit is axially displaced when applied to the structural component or during the drilling operation causing it to move opposite to the drilling direction. It is possible that the tip at the leading end of the centering bit may move opposite to the drilling direction into the hollow carrier member rearwardly of the drilling face of the core drill bit and, as a result, centering of the core drill bit can not be effected.